Beelzebub is a wise man, the words of reason for all those in Hell. The fallen in Hell are aware that Beelzebub is their prophet. the words of wisdom and future for their own future. Milton presents Beelzebub not as a parody, but as a true sage. While the whole fact that the members of hell are fighting God from the beginning is wrong morally, and the fact that the demons in Hell will lose is inevitable, this doesn't make Beelzebub and less logical or comically in how he addresses the members of Hell. Beelzebub is someone who is greatly respected and admired by those he is speaking to, including Satan. If he was not, Satan would not have him speak.
He stands to speak because this is a signal to the denizens of Hell that his great wisdom is about to be dispersed to those in Hell. To stand means that he is to command the attention and respect. Whenever it is alluded to Beelzebub standing in this section, this imagery of respect and dignity follows, a further attempt for Milton to show the reader the character of Beelzebub.
-Erik Scott